How To Size Headers

In the world of engine hop-up components, headers are truly a win-win-win deal. They can deliver measurably better torque, horsepower, and mileage. When swapping out factory iron manifolds for headers, a typical peak-to-peak gain (even for a smog-laden motor) is about 25 hp, and roughly the same for lb-ft of torque. Having said that, the low-speed torque can go up as much as 40 lb-ft with headers that are optimized for the engine. For the average hot rodder there is no downside, and for the racer they are essential. What they are not, though, is foolproof. Headers can deliver a great deal in terms of return on investment—if the headers are spec’d out properly. When it comes to the right spec, it’s all about dimensions.

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The critical dimensions we need to concern ourselves with are primary diameter, primary length, collector style, secondary diameter, and secondary length. The combination of all these dimensions working in concert is what is commonly referred to as a tuned system. When the system is tuned correctly not only does the exhaust leave the cylinders in an unhindered fashion, but also pressure waves below atmospheric pressure act on the cylinder during the valve overlap period to effectively scavenge the combustion chamber of the last remnants of the exhaust. This action, in turn, causes the intake charge to start into the cylinder well before the piston starts its induction stroke. In other words, there are in effect two induction events. The first is the exhaust-driven one, and the second is the piston-driven one.

3/9Shown here are the critical lengths and pipe diameters that need to be sized to suit the engine spec and the rpm band to be most used.

So how does this exhaust-driven induction stroke come about? In essence, it is relatively simple. When the exhaust valve is initially opened, the cylinder pressure is still relatively high at some 70 to 120 psi. Opening an exhaust valve with this kind of pressure behind it generates a very strong positive (high) pressure wave that travels down the exhaust pipe at the speed of sound (some 1,300 feet/second in a hot exhaust environment). When this positive pressure wave reaches the end of the pipe (or any substantial change in cross-sectional area), it is reflected and undergoes a reversal such that a positive pressure becomes a negative (below atmospheric) pressure wave.

4/9This chart works well for high-performance heads in as-cast or ported form. To determine the primary inside diameter, you will need a good ballpark figure for the exhaust flow at the peak valve lift. Using this figure, go up the chart until you reach one of the colored lines (green for street, purple for street/strip, blue for race). At that point go across the chart and read off the pipe ID required.

The negative pressure wave now travels back up the exhaust pipe, and if it arrives at the exhaust valve during the overlap period, it will suck the remaining exhaust out of the combustion chamber and start the intake charge moving into the chamber. The trick is to match lengths to the operating rpm of the engine. By coupling up cylinders and adding the effect of a secondary collector length, the range over which the exhaust can scavenge the cylinder can be made broader to the tune of some 4,000 rpm. With a well-tuned system, the degree to which the exhaust can “suck” on the cylinder to start the intake charge on its way can be very substantial. On a well spec’d high-performance street engine, on up to an all-out race engine, this suction can amount to as much as 7 psi, and that’s way more than the suction caused by the piston going down the bore.

Primary Sizing

Primary pipe length is usually considered to be the most important dimension to get right. While this may be so in most instances, it is not the case for a two-plane—cranked V-8 typical of Detroit products we commonly drive. The suspected reason for this is the unevenly spaced firing pulses seen at the collector, but whatever the reason, it does (for once) all work in our favor, as we’ll see later.

With primary length demoted from the most important dimension, the primary pipe diameter becomes the number-one dimension to get as near optimal as possible. The target, for a street motor, is to get the sizing such as to deliver the most area under the torque and power curve along with good fuel efficiency at part throttle. For a street/strip application the bias is toward making more top end while sacrificing a little low down. For a race engine, it’s all about power output in the top 1,500 to 2,000 rpm. There are a lot of formulas that address the sizing of the primaries, but a method that I’ve used to great effect over the last 20 years employs a chart that shows the correlation between exhaust port flow at the peak valve lift and primary pipe diameter. Use the chart in this story (shown above) and you’ll find sizing in this department to be really close.

5/9Here are some dyno test results of four different primary tube lengths on an 8.5:1 compression 350 Chevy. The shortest length at 18 inches (black curves) show the engine did not like really short primaries. But the 29-inch (green), the 32-inch (red) and the 38-inch (blue) headers all performed well with only a moderate bias toward better low end from the longer pipes.

With the diameter selection taken care of, it’s time to consider primary lengths. Here we are fortunate to have a wide operational window due to the two-plane crank configuration of a Detroit V-8. In broad terms we can say that shorter primaries favor top end output while longer ones are better for low and midrange. That said, dyno tests show that the variance in output with a primary length change is, over quite a wide range of lengths, small. In my testing of a small-block 350 Chevy, I found that if the primary lengths fall within a 29- to 38-inch range, there is little difference in output. In reality, this test did not show the full range of primary lengths that would work. For street use, the primaries can be as long as 46 inches, and yet will still show good (though not optimal) top end results. We can conclude that fixating on precisely (or even closely) equal primary length is not a necessary requirement for a good header.

Collector Design

6/9This is the conventional collector form, and it works well.

Just so we’re all on the same page, we’re going to define the collector as the part of the header that brings the four primaries together to form one pipe. After they have become one, the pipe itself will be referred to as the secondary pipe. In essence, we have three styles of collectors. First, we have the four-into-two-into-one collector that has been popular with NASCAR teams for the past decade or so. This type of collector rarely finds its way into the general hot rodding community, probably because of its cost and that it’s less easily installed. That said, it works, but the return on investment is generally small. This leaves us to consider the two most popular collector styles. The original simple parallel collector where the four pipes merge into a pipe the size of the intended secondary, and the merge collector where the four primaries come into a venturi-like form before expanding into the secondary pipe diameter.

Fortunately, there is not much to cover here. First, the original parallel collector works well, but my dyno tests indicate that the merge collector has a slight edge in terms of power bandwidth, torque, and horsepower. In another area, if the design is sufficiently well developed, the merge collector has the ability to pull more vacuum via the evac-u-pan system, if one is being used. Notwithstanding, the merge collector is a little more costly to make, so ultimately you will have to go with whatever suits your budget.

7/9A merge collector like this, however, often has the edge over a standard collector.

Secondary Sizing

As insensitive as the primary pipe lengths are for power, the opposite is true of the secondaries. Here we find that just a few inches can make the difference between winning and going out in the quarter-finals. You often see headers that have no secondary length of any consequence, just the short stub on the collector section, and that’s a mistake. For such a seemingly trivial piece of pipe, this can be the loss of a fair chunk of power. Just how much this can be in a relatively normal situation can be seen from tests I did by adding 12 inches of length to a typical stub collector. In that test an extra 12 hp and 10 lb-ft (peak to peak) were gained.

8/9NASCAR headers like this one typically employ a four-into-two-into-one collector. This typically results in about a 10 hp gain right at the top end of the rpm band.

When longer secondary lengths are used, the effect on power and torque is even more dramatic. The chart below demonstrates the difference among 21-, 16-, and 12-inch secondary lengths (as measured from the small end of the four-pipe merge). Obviously, longer secondary pipes are fine for the street, where low-speed torque is as important as anything, but if you’re racing only the top 1,500 to 2,000 rpm is used. If we take an average of the output over the range from 5,000 to 6,750 rpm, we see a very substantial difference in output, all from paying attention to a simple thing like the exhaust secondary length.

9/9Here we see the effect of adding a tuned length (about 12 extra inches) to the normal stub length seen on a regular collector. The addition of 12 hp and 10 lb-ft is good, but just check out the 40 lb-ft gain at 3,600 rpm.

The last dimension we have to look at is the secondary/collector diameter. This is a pretty simple aspect to deal with. For a street/strip or race application, a good rule of thumb is to multiply the primary pipe diameter by 1.8 and then select the nearest available pipe size. A little oversize won’t hurt much, but it’s worth noting that often selecting something a little undersize is better than a bigger increment oversize. For example, if your calculated secondary pipe size works out to be 1/16-inch more than one standard pipe size but 3/16 less than the next size up, then opt for the smaller pipe as results are likely to be much better. If mileage and street performance is your goal, then a secondary up to ½-inch less than predicted here can work well.

Most readers are likely to get their headers and systems from a company like Hooker. This is fine, but if you want something special you may want to calculate as much as possible the optimum system for your engine. In this regard, I’ve been reviewing a program by cylinder head and exhaust specialist Larry Meaux (www.MaxRaceSoftware.com) that so far has correlated well with what I have found works on the dyno. This program also computes the requirements for a muffled system.